1. Field of the Invention
The present invention relates to an image forming apparatus such as a copier, a printer, a facsimile machine, a plotter, a multi-function machine including at least two of these functions, and a digital direct reproducer, and a development device and a process cartridge used in the above-mentioned image forming apparatus.
2. Discussion of the Background
In general, an electronographic image forming apparatus, for example, a copier, a printer, a facsimile machine, etc., includes an image forming mechanism for forming an electrostatic latent image on an image carrier, developing the latent image with toner, transferring the developed image onto a recording medium, and fixing the image thereon.
Generally, a development device using two-component developer consisting essentially of toner and magnetic carrier includes a developer carrier and a developer-transporting member, both provided inside a development case having an opening facing the image carrier. The developer carrier is partially exposed from the opening, and carries the developer.
The developer-transporting member mixes and agitates the toner and the magnetic carrier in the development device, and transports the developer whose toner concentration is appropriately controlled. After the amount of the developer is set as appropriate by a developer regulator, the developer that is carried onto the developer carrier by the developer-transporting member is transported to a development area facing the image carrier. In the development area, the toner in the developer carried on the developer carrier is adhered to the electrostatic latent image formed on the image carrier to form a desired toner image.
In the development device performing an image forming operation, air flows in from the opening, and the developer carrier located at the position facing the opening and the developer-transporting member are revolved to generate airflow inside the development device. This airflow causes internal pressure of the development device to vary locally.
Most of the toner in the development device is adsorbed onto the magnetic carrier electrostatically due to frictional charging with the magnetic carrier. As noted above, however, in the development area, an electrical field generated between the image carrier and the development carrier causes the toner attracted onto the magnetic carrier to adhere instead to the electrostatic latent image on the image carrier. In other words, the strength of the electrical field exerted on the toner exceeds the electrostatic force attracting the toner to the magnetic carrier, and therefore the toner leaves the magnetic carrier and flies to the side of the image carrier.
However, the toner, which is a powder, does not have a consistent and uniform shape and consequently some toner have an insufficient ability to accept charge. If as a result the toner is distributed unevenly and remains in a part of the development device, such toner may be insufficiently charged because the frictional charging with the magnetic carrier may be insufficient.
Moreover, the magnetic carrier in the developer inside the development device loses its charging capability over time with repeated usage of the developer, and then fails to charge the toner sufficiently. Since insufficiently charged toner has weak attractive power and since the toner is light, the toner can be scattered by the airflow generated inside the development and scatter.
As the internal pressure of the development device increases, it can scatter the toner from openings in the components of the development device all over the interior of the image forming apparatus, contaminating the image forming apparatus.
Several proposals have been made to prevent the toner form escaping from the development device and scattering. For example, in known techniques, a development device includes a discharging port provided on an upper surface thereof that reduces internal pressure by discharging air from the development device, and a developer-supporting member, like a filter, that is provided on the discharging port catches developer that does escape, so that scattering of toner from the development device is prevented or reduced.